It is well known that a powder coating is less volatile and malodorous than a solvent coating and thus is very useful from the standpoint of environmental protection and regulation.
Powder coatings which have heretofore been commercially available have a volume average particle size of 30 to 40 .mu.m and a broad particle size distribution because of rough classification.
On the other hand, the market demands powder coatings that exhibit an improvement of leveling, that possess a reduced film thickness so as to enhance working efficiency, and that decrease the total cost, etc.
Films having conventional powder coatings cannot exhibit a good leveling unless the films contain two or more particle layers. Thus, the thin layer of a film having a conventional powder coating has a thickness of about 50 .mu.m at minimum. Further, because the conventional powder coating has a broad particle diameter distribution, it can easily cause roughness on the film surface due to coarse particles. Thus, no prior art powder coatings can meet the above-mentioned market demands yet.
Further, when an electrostatic apparatus, which is known in the art for powder coating processes, is used for coating, the surface to be coated attracts highly chargeable particles (those having a large particle diameter) more easily than others. The unapplied and recycled fractions of the powder coating quite differ in properties from the original product. When re-used, the fractions thus recycled cause a drop in coating efficiency and film properties.